The present invention relates to an actuator and, particularly, to a laminated-type piezoelectric actuator having strain gages.
A piezoelectric actuator which utilizes the piezoelectricity of crystal has been used widely in a high precision positioning mechanism since it can control a minute mechanical displacement at high speed. Among others, a laminated-type piezoelectric actuator composed of a laminated structure of piezoelectric films of piezoelectric ceramics, etc., each having thickness of several tens microns and having a thin film internal electrode provided thereon is featured by its capability of generating large force with relatively low drive voltage and has been widely utilized in the fields of high precision positioning control of a semiconductor manufacturing apparatus such as exposure apparatus, control of valves of a mass flowmeter, thickness control in an extruder of plastic film and optical axis control of an optical device, which require high precision control of minute displacement in the order of micron or less.
Since an amount of displacement of such laminated-type piezoelectric actuator depends upon strain generated in the respective piezoelectric ceramics layers correspondingly to a drive voltage applied externally across the respective piezoelectric ceramic layers through external electrodes, it is basically possible to control the amount of displacement of the piezoelectric actuator by controlling magnitude of the drive voltage. In the case of actuator utilizing piezoelectric effect, however, a relation between drive voltage and strain of respective piezoelectric material (ceramic) layers is usually not linear but exhibits hysteresis. Therefore, when the piezoelectric actuator is used in these fields to precisely control displacement in the order of micron or less, it is necessary to detect an actual displacement of the piezoelectric actuator and feed it back to the drive voltage. A construction of such piezoelectric actuator having a strain gage for detecting an amount of displacement of the actuator is disclosed in Japanese Utility Model Laid-open No. Sho 61-140661.
FIG. 1 is a schematic perspective view of a conventional piezoelectric actuator having a strain gage. The piezoelectric actuator itself includes an active portion 2 shown in FIG. 2. Usually, the piezoelectric actuator includes an upper protective layer 31 and a lower protective layer 32 provided on opposite ends of the active portion 2, respectively, as shown in FIG. 2 for reasons of use. In FIG. 2, the active portion 2 is constituted with an alternatively laminated structure of a plurality of piezoelectric ceramics layers 4 and a plurality of internal electrode layers 5. The internal electrode layers 5 are connected to external electrodes 71 and 72 alternatively to form a pair of comb electrodes having electrode fingers arranged interdigitally in cross section, so that the external electrodes 71 and 72 function as opposing electrodes opposing each other through the piezoelectric ceramics layers 4. In FIG. 2, a drive voltage applied between the external electrodes 71 and 72 through leads 711 and 712 is applied between adjacent internal electrode layers 5 to form an electric field across each piezoelectric ceramics layer 4 sandwiched between the internal electrodes. With such electric fields, the respective piezoelectric ceramics layers 4 generate strain in a direction perpendicular to a plane of the piezoelectric ceramics layer, that is, in a thickness direction or laminating direction. The piezoelectric actuator derives the strain as a change of length of the laminated-structure in a thickness direction, that is, a displacement, and transmits it externally.
In the piezoelectric actuator shown in FIG. 1, a strain gage 8 is attached onto a side surface of the actuator on which there is no external electrode is provided. When the drive voltage is applied between the external electrodes 71 and 72, the piezoelectric actuator is extended in the laminating direction thereof by a predetermined amount due to piezoelectric effect. With such expansion of the piezoelectric actuator in the thickness direction thereof, a resistor of the strain gage 8 is subjected to tensile force and its resistance value is increased. An amount of the resistance change is detected through leads 9 and which is linearly related to an amount of expansion or contraction of the piezoelectric actuator, that is, an amount of displacement. Therefore, by preliminarily measuring a relation between the amount of displacement of the piezoelectric actuator and the amount of resistance change of the strain gage at respective drive voltages and detecting the amount of resistance change of the strain gage 8 when a predetermined voltage is applied between the external electrodes 71 and 72, it is possible to know the amount of displacement of the piezoelectric actuator. Further, when the amount of resistance change of the strain gage 8 is inconsistent with an aimed setting value, it is possible to control the amount of resistance change to the aimed displacement by controlling the drive voltage correspondingly to a deviation of the amount of resistance change from the aimed value.
As mentioned above, the amount of expansion or contraction, that is, the amount of displacement, of the laminated-type piezoelectric actuator can be controlled precisely by detecting an actual amount of displacement by means of the strain gage attached to the side surface of the laminated structure and feeding back a difference between the detected displacement and the aimed displacement to the drive voltage.
However, in case where a laminated-type piezoelectric actuator is to be used practically, in addition to the laminated active portion for generating displacement corresponding to the externally applied drive voltage, portions such as protective layers for protecting the active portion and temperature compensating members of metal for improving accuracy of displacement control, which do not produce displacement with the drive voltage, are required in the laminated structure. In such conventional actuator as mentioned above, there may be a difference between a total amount of actual displacement of the whole piezoelectric actuator and an amount of displacement detected by the strain gage, upon which it may become impossible to control displacement precisely. This will be described in detail below.
As mentioned previously, FIG. 2 shows the cross section of the laminated-type piezoelectric actuator having the above-mentioned protective layers. In FIG. 2, the piezoelectric actuator includes a laminated-structure 10 of the active portion 2 and the protective layers 31 and 32 as its basic components.
The active portion 2 is constituted with the alternatively laminated structure of the piezoelectric ceramics layers 4 and the internal electrode layers 5 and displacement thereof is generated in the laminating direction by the drive voltage applied to the external electrodes 71 and 72 and hence to the respective piezoelectric ceramics layers 4 through the respective internal electrodes 5.
The protective layers 31 and 32 are provided to protect the active portion 2 electrically and mechanically against external force. That is, although the basic function of the piezoelectric actuator as an electromechanical transducer is obtained by the active portion 2, it is desirable, in order to use it on a practical device, that an outermost portion of the laminated-structure 10 in the laminating direction is of an insulating material since it can be adapted to an associated device even if the latter is formed of not insulating material but metal material. Further, since each piezoelectric ceramics layer 4 of the active portion 2 is as thin as several tens microns and strength of the electric field generated in the piezoelectric ceramics layer 4 by the drive voltage in the order of 150 V applied thereto is very large, the active portion 2 must be protected mechanically against external mechanical shock in such a way that the piezoelectric ceramics layer or layers 4 are not cracked or damaged. For this purpose, it least the protective layers 31 and 32 are provided on an upper and lower ends of the active portion 2 and, therefore, each protective layer should be thick enough to provide desired protection. For example, the thickness, that is, length in the laminating direction, of each protective layer may be in the order of 2 mm for the active portion 2 having length of 12 mm in the laminating direction. In view of easiness of fabrication of the protective layers 31 and 32, each protective layer is usually formed by laminating thin layers of the same material as that of the piezoelectric ceramics layer 4 constituting the active portion 2.
In the laminated-type piezoelectric actuator constituted as mentioned above, when external force exerted on the laminated structure 10 in the strain generating direction, that is, the laminating direction, is varied, elastic strain of the protective layers 31 and 32 is different from that of the active portion 2 even if they have the same piezoelectric characteristics, since the active portion 2 is subjected to electric field while the protective layers are not. That is, an actual amount of displacement of this laminated structure is a sum of strain of the active portion 2 and strain of the protective layers 31 and 32. In the piezoelectric actuator shown in FIG. 1, however, strain of only the active portion 2 is detected. Therefore, the detected displacement is inconsistent with the actual displacement. Further, since the length of the protective layers 31 and 32 is not negligible with respect to the length of the active portion 2 as mentioned previously, the difference between the actual displacement and the detected displacement is very important.
On the other hand, when temperature of the laminated structure 10 is changed by change of ambient temperature and/or heat generated by an operation of the piezoelectric actuator, detected displacement also becomes inconsistent with actual displacement if coefficient of linear expansion or the active portion 2 is different from that of the protective layers 31 and 32.
It may be considered, in order to flatten temperature characteristics of displacement amount of the piezoelectric actuator, to further provide a temperature compensating member on the upper or lower portion of the laminated structure 10. Such temperature compensating member is a block of such as stainless steel whose coefficient of linear expansion is opposite in sign to that of the laminated-structure 10 formed of piezoelectric material whose coefficient of linear expansion is negative and functions to compensate for variation of displacement of the piezoelectric actuator due to thermal expansion thereof. Length of the temperature compensating member in the laminating direction may be preferably in the order of about 4 mm for the laminated-structure 10 having length of 16 mm. Even in such piezoelectric actuator having such temperature compensating member, a detected displacement also becomes inconsistent with an actual displacement when elastic strain of the active portion 2 is different from that of the protective layers 31 and 32, resulting in degraded accuracy of displacement control.
That is, when a piezoelectric actuator includes, in addition to an active portion which generates displacement corresponding to a drive voltage, portions whose elastic strains and/or strains due to thermal expansion are different each other, preciseness of displacement control according to the conventional technique tends to be degraded when external force in a laminating direction and/or temperature is changed. Such degradation of displacement control accuracy also occurs by change of drive voltage when the active portion is constituted with a plurality of portions made of materials whose piezoelectric properties are different from each other.